doc: Add explanation that -alt-grab and -ctrl-grab affect special keys

Phillip Merensky reported that the special keys (e.g. Ctrl-Alt-f for full
screen) did not work correctly if -alt-grab is used.

BUG: 696530

Review of ui/sdl.c:sdl_refresh indicates that this is the intended behaviour,
so we should update the documentation to match the actual behaviour, as
suggested by Phillip in the bug report.

Signed-off-by: Brad Hards <bradh@frogmouth.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

qemu-doc.texi

@@ -278,7 +278,11 @@ targets do not need a disk image.
 
 @c man begin OPTIONS
 
-During the graphical emulation, you can use the following keys:
+During the graphical emulation, you can use special key combinations to change
+modes. The default key mappings are shown below, but if you use @code{-alt-grab}
+then the modifier is Ctrl-Alt-Shift (instead of Ctrl-Alt) and if you use
+@code{-ctrl-grab} then the modifier is the right Ctrl key (instead of Ctrl-Alt):
+
 @table @key
 @item Ctrl-Alt-f
 @kindex Ctrl-Alt-f

qemu-options.hx

@@ -662,7 +662,8 @@ DEF("alt-grab", 0, QEMU_OPTION_alt_grab,
 STEXI
 @item -alt-grab
 @findex -alt-grab
-Use Ctrl-Alt-Shift to grab mouse (instead of Ctrl-Alt).
+Use Ctrl-Alt-Shift to grab mouse (instead of Ctrl-Alt). Note that this also
+affects the special keys (for fullscreen, monitor-mode switching, etc).
 ETEXI
 
 DEF("ctrl-grab", 0, QEMU_OPTION_ctrl_grab,
@@ -671,7 +672,8 @@ DEF("ctrl-grab", 0, QEMU_OPTION_ctrl_grab,
 STEXI
 @item -ctrl-grab
 @findex -ctrl-grab
-Use Right-Ctrl to grab mouse (instead of Ctrl-Alt).
+Use Right-Ctrl to grab mouse (instead of Ctrl-Alt). Note that this also
+affects the special keys (for fullscreen, monitor-mode switching, etc).
 ETEXI
 
 DEF("no-quit", 0, QEMU_OPTION_no_quit,